gopls/internal/regtest: use a real file system notifier

[findleyr]

It looks like several gopls testing flakes may be related to inaccurate file system notifications (e.g. #51930).

Historically, the gopls regtests had executed in the x/tools module, and so could not import cross platform libraries for filesystem notifications (e.g. https://github.com/fsnotify/fsnotify). To work around this, file watching is 'faked' via polling.

The regtests are now in x/tools/gopls, and we should consider replacing this polling with an actual notification library. This would theoretically reduce flakes and improve accuracy.